A typical architecture for an integrated receiver is a zero IF(ZIF) architecture in which a received signal is split into two paths and applied to respective mixers to which are applied a local oscillator signal having a frequency of such a value as to frequency down convert the received signal to a ZIF, A relative phase shift of approximately 90.degree. exists between the local oscillator signals applied to the respective mixers. The signal path from one of the mixers is designated an in-phase or I channel and that from the other of the mixers is designated as a quadrature or Q channel. The products of mixing in the I and Q channels are low pass filtered to derive the difference components which are thereafter demodulated.
A well known problem with ZIF receivers is that of dc offsets, which occur due to the various stages, are directly coupled. Other problems occur due to some of the wanted products of mixing being at dc or close to it. The occurrence of dc offsets within the various stages leads to severe desensitizing the receiver circuit which inhibits direct coupling of these stages. Techniques for avoiding the effects of dc offsets include providing ac coupling circuits or more sophisticated dc blocking circuits between successive stages. Inevitably this can lead to loss of some of the wanted products of mixing. In the art, what is termed a notch is created in the frequency response on or about the ZIF. The width of the notch is typically a function of the capacitance value used in the ac coupling circuits. Thus to minimise the width of the notch, a high value capacitance is used. However the higher the value of capacitance the longer the ac-coupling time constant which in turn places a limit on the speed with which the receiver can recover from a change in dc offset caused by amplitude modulated or pulsed interfering signals. While the ac-coupling time constant can be reduced by using a lower value of capacitance, this causes the width of the notch to be greater thereby eliminating more of the wanted signals.
U.S. Pat. No. 4,944,025 discloses a direct conversion FM receiver that includes ac-coupling and automatic gain control. The receiver receives a signal at a radio frequency, frequency down converts the received signal and filters the signal which is subsequently frequency up-converted and then demodulated. The frequency-down conversion, filtering and up-conversion is done using quadrature related mixers the respective outputs of which are successively ac-coupled to an amplifier, a low pass filter and a mixer. In order to overcome the problem of the notch in the frequency spectrum due to ac-coupling, the local oscillator frequency supplied in quadrature to the frequency down-converting mixers is offset from the nominal carrier frequency of the received signal by the sum of the width of the notch plus the base bandwidth of the modulated signals. In a numerical example the offset corresponds to about a quarter of the channel spacing. The bandwidth of the low pass filters is such as to pass the band of the wanted signals but block signals on adjacent channels. Although the invention disclosed in U.S. Pat. No. 4,944,025 goes some way to mitigating the problem due to the notch created by ac coupling, there are nevertheless constraints on the choice of value of the ac-coupling capacitors because if it is relaxed too far, inadequate image rejection occurs because as the offset frequency, which is a function of the size of the notch, increases the adjacent channel signals will be closer in frequency to the wanted signals and will be more difficult to separate by simple filtering.
Another problem which occurs with direct conversion receivers, and is not considered by U.S. Pat. No. 4,944,025 is that of the presence of strong amplitude modulated signals in the rf signal bandwidth, which signals are directly detected and appear in the ZIF band and cannot be removed by filtering.
It is an object of the present invention to be able to relax the ac-coupling constraints in an integrated direct conversion receiver.
According to the present invention there is provided a receiver comprising means for frequency down-converting a received signal to produce quadrature related i.f. signals at a frequency offset from d.c, means for blocking dc-offsets in the quadrature related i.f. signals, means for recovering the wanted i.f. signals and means for equalising the wanted i.f. signals and detecting modulating signals present in the i.f. signals.
The present invention is based on the realisation that the signal equaliser will attempt to overcome all the distortion occurring in the path of the signal from the point when it leaves the transmitter until it enters the equaliser. Hence it is possible to increase the width of the notch by using a low value capacitor and thereby reducing the effect of the ac coupling time constant on the recovery of the receiver from a change in dc offset caused by amplitude or pulsed interfering signals while recovering the modulating signal substantially undistorted.
The receiver may have various architectures such as a phasing receiver or a polyphase receiver.
The amount of the frequency offset may be chosen to place any known interfering signals at one edge of, or beyond, the bandwidth of the wanted signals.
In an embodiment of the present invention the means for recovering the wanted i.f. signals comprises polyphase filtering means, which may be implemented as gyrator filtering means or as transconductor integrator filtering means, and which is able to provide a complex output signal in response to a complex input signal and can effectively filter out the image of the wanted signal.